Switch-mode power supplies are used in many applications for supplying power from e.g. an AC mains net or a DC voltage source to electric or electronic devices that typically need a regulated voltage supply. An example of such a device is an actuator system with a number of linear electric actuators. Such actuator systems are typically used in adjustable furniture, such as adjustable tables, beds and chairs, but linear electric actuators are also used in several other industrial products, where they can be integrated into a mechanical structure for adjustment of a mechanical movable component. In these applications, switch-mode power supplies have advantages regarding low losses and compact structure.
Many switch-mode power supplies used in these applications are based on a flyback converter, which is a converter type with galvanic isolation between input and output that can be used in both AC/DC and DC/DC conversion. If the flyback converter is used in AC/DC conversion, a bridge rectifier and a capacitor are arranged between the AC mains supply and the converter itself. The main component of the flyback converter is a flyback transformer, the primary side of which is connected to a controllable switching element, typically in the form of a MOSFET transistor, arranged to switch the primary current in the transformer on and off. The secondary winding of the transformer is connected through a diode to a capacitor in parallel with the load, i.e. the device powered by the power supply.
When the switching transistor is in its conducting state, the primary side of the transformer is connected to the input voltage so that energy is accumulated in the transformer, while on the secondary side, the diode is reverse biased, thus preventing a secondary current from flowing. When the switching transistor is in its non-conducting state, no primary current can flow, while on the secondary side, the diode is forward biased, thus allowing a secondary current to flow. In this stage, energy accumulated in the transformer during the previous stage is now transferred from the transformer to the secondary side.
The output voltage is usually controlled by a pulse width modulation control circuit in dependence of the power consumed by the powered device by turning the switching transistor on and off at a certain rate and a certain duty cycle based on a galvanic isolated feedback signal related to the output voltage.
Since the flyback transformer is not ideal, it will have a certain stray inductance or leakage inductance that can be considered as a small inductor in series with the primary winding. Although the energy accumulated in the transformer when the switching transistor is conducting, is transferred from the primary side to the secondary side when the switching transistor is turned off, a small amount of energy accumulated in the leakage inductance will not be transferred. The opening of the switching transistor will therefore cause a high and sharp voltage peak over the transistor, as it will always be the case when an inductor is suddenly disconnected from a DC current. Actually, the leakage inductance will form a series resonant circuit with the parasitic source-drain capacitance of the switching transistor and cause a damped high frequency oscillation.
This high frequency oscillation can be a source of electromagnetic interference in other circuits, which then need to be protected against such interference. Further, the voltage over the switching transistor can reach a level so high that the switching transistor may be damaged or destroyed, or a more expensive transistor type having a voltage rating that exceeds this higher voltage level must be used.
These problems may be counteracted by a so-called snubber circuit, which provides a short-term alternative current path around the switching transistor so that the leakage inductance can be discharged more safely by removing the amount of energy accumulated in the leakage inductance.
A known example of such a snubber circuit consists of a snubber diode in series with a parallel combination of a snubber capacitor and a resistor. The leakage inductance current can now find a low impedance path through the snubber diode and the snubber capacitor so that leakage inductance energy is transferred to the snubber capacitor and then dissipated in the resistor.
Although such snubber circuits do prevent the high voltage spike over the switching transistor from occurring, the power lost in the circuits due to the energy dissipated in the resistor reduces the overall efficiency of the flyback converter.
Lossless snubber circuits capable of recycling the energy stored in the snubber capacitor to the primary winding of the transformer are also known. One example is disclosed in US2009268489 A1 to FSP Technology Inc. Typically, the known lossless snubber circuits need to be able to adapt to a varying load situation, which is achieved by using a timer circuit synchronized to the frequency and duty cycle of the pulse width modulation control circuit. However, the implementation of such circuits is quite complex because of a high component count, and consequently also a high cost.